mdoc.su: add a BSD licence, as per /usr/share/misc/license.template
mdoc.su: better variable names for fp,np,op: fr,nr,or (no code flow changes)
mdoc.su: support multi-flavour URLs like http://mdoc.su/f,d/ifnet.9 and even http://mdoc.su/f60,n40,o37,d/ral.4 and http://mdoc.su/FreeBSD-8.2,NetBSD-6.0,OpenBSD-4.7,DragonFly/aibs.4
mdoc.su: support /fd/ifnet.9 for multi-flavour, in addition to http://mdoc.su/f,d/ifnet.9

This is experimental:  if you can type the extra comma, it'll probably
be a better and more unambiguous abbreviation.